A new documentation is available for Squash TM 2.0 and later versions, check it here
The link between a JIRA sub-task and its parent task is represented by a parent-subsidiary requirement link in Squash TM, only if both requirements are displayed in Squash TM, within the same synchronization.
In JIRA, this link has a specific meaning, it is subsequently not recommended to disrupt the hierarchy between a task and its sub-tasks, in order to obtain a correct reporting of the parent requirement.